1. Field of the Invention
The present invention relates to an image processing method for forming an image output condition of an image output unit on the basis of read data of reference image output from the image output unit, an image processing apparatus for executing the above image processing method, and a recording medium for storing a program to realize the above image processing method.
2. Related Background Art
In recent years, various peripherals such as a personal computer, a printer and the like come into popular use, whereby everyone can easily output hard copies of a word processing document and a graphic document formed on the computer.
As a typical example of such a structure, a system as shown in FIG. 10 has been known.
That is, FIG. 10 shows the schematic structure of the system in which a page layout document, the word processing document, the graphic document and the like are formed based on DTP (desktop publishing) using a host computer 101, and the formed document is output as the hard copy by a laser beam printer, an ink jet printer or the like.
In FIG. 10, numeral 102 denotes an application which operates on the host computer 101. As the typical applications, word processing software such as “WORD” available from Microsoft and page layout software such as “PageMaker” available from Adobe are widely used.
A digital document formed by using such the software is transferred to a printer driver 103 through a not-shown OS (operating system) of the computer.
Ordinarily, the digital document is represented as gathering (or aggregate) of command data of figures, characters and the like constituting one page, and such the command data are transferred to the printer driver 103. A series of commands constituting a screen are represented in the language called as PDL (page description language). A GDI, a PostScript and the like are widely used as the typical PDL.
The printer driver 103 transfers the received PDL command to a rasterizer 105 in a raster image processor 104. The rasterizer 105 expands the character, the figure and the like represented based on the PDL command to a two-dimensional bit map image in order to actually print output the character, the figure and the like. In the bit map image, a two-dimensional plane is filled with repetition of one-dimensional rasters (i.e., lines). This is the reason why the unit to form the bit map image is called as the rasterizer. The expanded bit map image is temporarily stored in an image memory 106.
FIG. 11 schematically shows the above operation. That is, a document image 111 displayed on the host computer 101 is transferred as a PDL command queue 112 to the rasterizer 105 through the printer driver 103, and the rasterizer 105 expands a two-dimensional bit map image 113 onto the image memory 106.
The expanded image data is transferred to a color printer 107. Since the printer 107 contains an image formation unit 108 of known electrophotographic system or ink jet recording system, the printer 107 forms a visible image on a paper sheet by using the unit 108 and then print outputs the formed visible image. Of course, the image data in the image memory 106 is transferred in synchronism with a not-shown sync signal or clock signal necessary to operate the image formation unit 108, a transfer request of a specific color component signal and the like.
In the above-explained conventional example, it has been known that following various problems occur as to the image formation unit used for print output.
That is, if the identical original document is repeatedly read and output, a tint of the output image finally obtained from the identical document often varies every time the image is output due to unstableness of an output characteristic of the image formation unit and dispersion between the connected devices. Further, if the output image is printed by different printers, the result obtained by one printer is often different from that obtained by other printers.
The reason why such inconvenience occurs is as follows. For example, it is assumed that the image formation unit applies the electrophotographic system. In an electrophotographic process of this system, various processes such as laser exposure, latent image formation onto a photosensitive body, toner developing, toner transfer onto a paper medium, toner heat fixing and the like tend to be influenced by ambient temperature and humidity, a time change of the parts (i.e., deterioration due to time elapse), and the like. Thus, a toner quantity finally fixed onto the paper medium varies every time the electrophotographic process is executed.
It has been known that such unstableness is not characteristic or peculiar to the electrophotographic system, but arises also in the inkjet recording system, a heat-sensitive transfer system and other various systems.
In order to eliminate such inconvenience, a system as shown FIG. 12 has been conventionally thought. In this system, a test pattern image 121 is output from the color printer 107, and the density of the output pattern is measured to correct the characteristic of the image formation unit 108. Hereinafter, the operation at this time will be explained in due order.
Initially, the host computer 101 sends a command for outputting a predetermined gradation pattern, to the raster image processor 104 (step (11)). The processor 104 forms a bit map pattern for the print output on the basis of the given command, and then transfers the formed bit map pattern to the color printer 107 (step (12)). Then the printer 107 prints or outputs the given bit map pattern onto the paper medium (step (13)). In this case, as shown in FIG. 12, the output test pattern image 121 includes four kinds of color pattern groups (i.e., C (cyan) pattern group 122, M (magenta) pattern group 123, Y (yellow) pattern group 124 and K (black) pattern group 125) respectively corresponding to four-color toners of the printer 107. Further, each color pattern group further includes patterns of eight levels “0” to “7” respectively corresponding to toner adhesion area ratios 0% to 100%.
Namely, the output test pattern image 121 includes total 32 rectangle print areas or patches (four colors×eight levels) each corresponding to the above minimum color pattern. A reflection densitometer 126 measures the density of each patch (step (14)), and then transfers the measured density value of each patch to the host computer 101 (step (15)).
Then the host computer 101 compares the measured density value with a previously stored reference value to form a correction table for each of the C, M, Y and K pattern groups 122 to 125, and then registers the formed tables in a not-shown table conversion unit of the raster image processor 104 (step (16)). Although there is no explanation, the table conversion unit is the table which is used to correct the value to be written as the bit map data in the processor 104 in case of forming the bit map image.
For example, on the output test pattern image 121, if the density of the third patch (level 2) in the C pattern group 122 is measured and judged to be lower than the reference value, the bit map data corresponding to this third patch is corrected to have a higher value in the correction table, whereby it is possible to set the density characteristic of the printer 107 closing up to the reference value.
By the above procedure, it is possible to stabilize the output density characteristic of the printer 107. However, as shown in FIG. 12, the densitometer to measure the printer output patch is necessary if doing so. Generally, since such the densitometer is very expensive, only a small number of users can buy it for exclusively stabilizing the density.
Further, even if the densitometer can be used, since the numerous patches of the printer must be sequentially measured one by one, it is laborious and cumbersome for the user.
Further, it has been thought a system in which a simple image input apparatus called a flatbed scanner is used instead of the expensive densitometer, and the output patches of the printer are read by this scanner to measure the density of each patch. In this case, however, since reading accuracy of the scanner is insufficient, if the patch is put at a different position on the original support board of the scanner, the measured and obtained density value seriously varies.